The basic prism light guide luminaire concept was first described by Whitehead in U.S. Pat. No. 4,615,579 issued Oct. 7, 1986 and was based on his prism light guide concept as described in U.S. Pat. No. 4,260,220. Since then many refinements to the prism light guide luminaire concept have been introduced, all of which are based on the principle that a prism light guide luminaire conducts light efficiently through a hollow air space by means of total internal reflection at the external surfaces of the guide, while allowing a pre-determined amount of light to escape at pre-determined points on the surface of the guide.
For example, U.S. Pat. No. 4,937,716 (Whitehead) describes how one may cause a prism light guide luminaire to emit light at a pre-determined rate which varies as a function of position on the surface of the guide. Briefly, light rays falling within the guide's acceptance angle encounter an internal diffusely reflecting surface which redirects a substantial portion of incident light into directions which cannot be confined within the guide by total internal reflection. Such redirected light rays are accordingly able to escape from the guide.
All such prior art prism light guide luminaires are encumbered by the fact that the light they emit is substantially diffuse. This limits the application of prior art prism light guide luminaires to situations in which diffuse light output is desirable, such as the many situations in which one might otherwise use fluorescent lamps. However, there are some situations in which it is preferable for a light source to emit somewhat collimated light. Examples include high bay lighting where it is important that light be efficiently directed downwardly from a high altitude location onto an illuminated surface; and, external sign illumination where light must be efficiently directed upwardly, through a restricted range of angles, onto the sign. It is an object of this invention to provide a prism light guide luminaire capable of efficiently emitting directed light in a manner which is practical to manufacture.
Prior art attempts to achieve directional light output from prism light guide luminaires have met with only limited success. One example is described in U.S. Pat. No. 4,850,665 (Whitehead) in which carefully oriented partially reflective, partially transmissive elements are mounted within the guide to reflect a pre-determined portion of the light passing through the guide into directions which the guide is unable to confine by total internal reflection, thus allowing such light to escape from the guide. Moreover, the direction of the reflected light is such that the natural refraction effect of the prismatic material used to form the guide further directs the escaping light into a pre-determined direction.
Alternate approaches are described in U.S. Pat. Nos. 4,984,144 and 4,989,125 (Cobb, Jr. et al). Portions of the prism light guide wall are removed and replaced with prismatic films which allow light to escape and which re-direct the escaping light into specific desired directions.
The prior art solutions aforesaid are subject to two fundamental problems. First, they require complex manufacturing and design techniques. It takes considerable effort to properly position and retain elements of the sort described by Whitehead in U.S. Pat. No. 4,850,665. Similarly, considerable effort is required to segment prism light guide wall material in accordance with the inventions of Cobb, Jr. et al. Moreover, these prior art approaches cannot collimate the escaping light to a degree which is substantially greater than the degree of collimation of the light within the prism light guide itself (typically .+-.30.degree.). Fundamental thermodynamic limitations should allow substantially greater collimation of the emitted light, since the guide's light emitting area is usually much larger than the area through which light enters the light guide from the light source.